Zinc alloy plated steel sheet having excellent bending workability and manufacturing method therefor

ABSTRACT

Provided are a zinc alloy plated steel sheet and a method for manufacturing the zinc alloy plated steel sheet. The zinc alloy plated steel sheet includes a base steel sheet and a zinc alloy plating layer, wherein the zinc alloy plating layer includes a Zn single phase structure as a microstructure and a Zn—Al—Mg-based intermetallic compound, and the Zn single phase structure has a degree (f) of (0001) preferred orientation expressed by Formula 1 below within a range of 50% or greater. [Formula 1] f(%)=(I basal /I total )×100 where I total  refers to the integral of all diffraction peaks of the Zn single phase structure when an X-ray diffraction pattern is measured within a range of 2 theta from 10° to 100° using a Cu-Kα source, and I basal  refers to the integral of diffraction peaks of the Zn single phase relating to a basal plane.

TECHNICAL FIELD

The present disclosure relates to a zinc alloy plated steel sheet havinghigh bending workability and a method for manufacturing the zinc alloyplated steel sheet.

BACKGROUND ART

A zinc plating method for suppressing the corrosion of iron by cathodicprotection has high anti-corrosion efficiency and economic feasibility,and thus has been widely used in manufacturing steel materials havinghigh corrosion resistance. Particularly, hot-dip zinc plated steelsheets, manufactured by dipping a steel material into molten zinc toform a plating layer, are obtainable through simple manufacturingprocesses and are relatively inexpensive, as compared to electro-zincplated steel sheets, and thus, demand therefor has increased in a widerange of industries, such as the automotive industry, the home applianceindustry, and the construction industry.

When a hot-dip zinc plated steel sheet is exposed to a corrosiveenvironment, zinc having a lower oxidation-reduction potential than ironundergoes corrosion first, and thus, corrosion of the steel sheet issuppressed by sacrificial corrosion protection. Along with this, compactcorrosion products are formed on the surface of the steel sheet as zincof a plating layer is oxidized, thereby protecting the steel sheet fromthe corrosive environment and improving the corrosion resistance of thesteel sheet.

However, air pollution and corrosive environments have increased withindustrial advances, and regulations on resource and energy savings havebeen tightened. Therefore, the need to develop a steel material havinghigher corrosion resistance than existing zinc plated steel sheets hasincreased.

In this regard, research has been variously conducted into techniquesfor manufacturing zinc alloy-based plated steel sheets having corrosionresistance improved by adding elements such as aluminum (Al) andmagnesium (Mg) to a zinc plating bath. Techniques for manufacturing aZn—Al—Mg-based zinc alloy plated steel sheet, which is representative ofzinc alloy-based plated steel sheets and manufactured by additionallyadding magnesium (Mg) to a Zn—Al plating composition, have been activelyresearched.

However, such a Zn—Al—Mg-based zinc alloy plated steel sheet has poorbending workability. That is, the zinc alloy plated steel sheet includeslarge amounts of Zn—Al—Mg-based intermetallic compounds in a platinglayer thereof as a result of thermodynamic reaction between zinc (Zn),aluminum (Al), and magnesium (Mg), and such intermetallic compounds maycause cracks in the plating layer during a bending process because ofhigh hardness of the intermetallic compounds, thereby lowering thebending workability of the zinc alloy plated steel sheet.

DISCLOSURE Technical Problem

Aspects of the present disclosure may provide a zinc alloy plated steelsheet having high bending workability and a method for manufacturing thezinc alloy plated steel sheet.

The present disclosure is not limited to the above-mentioned aspects.Other aspects of the present disclosure are stated in the followingdescription, and the aspects of the present disclosure will be clearlyunderstood by those of ordinary skill in the art through the followingdescription.

Technical Solution

According to an aspect of the present disclosure, a zinc alloy platedsteel sheet may include a base steel sheet and a zinc alloy platinglayer, wherein the zinc alloy plating layer may include a Zn singlephase structure as a microstructure and a Zn—Al—Mg-based intermetalliccompound, and the Zn single phase structure may have a degree (f) of(0001) preferred orientation, expressed by Formula 1 below, within arange of 50% or greater,

f(%)=(I _(basal) /I _(total))×100  [Formula 1]

where I_(total) refers to an integral of all diffraction peaks of the Znsingle phase structure when an X-ray diffraction pattern is measuredwithin a range of 2 theta from 10° to 100° using a Cu-Kα source, andI_(basal) refers to an integral of diffraction peaks of the Zn singlephase structure relating to a basal plane.

According to another aspect of the present disclosure, a method formanufacturing a zinc alloy plated steel sheet may include: preparing azinc alloy plating bath including magnesium (Mg) and aluminum (Al);obtaining a zinc alloy plated steel sheet by dipping a base steel sheetinto the zinc alloy plating bath to plate the base steel sheet; wipingthe zinc alloy plated steel sheet with gas to adjust a plating weight;and after adjusting the plating weight of the zinc alloy plated steelsheet, cooling the zinc alloy plated steel sheet by spraying droplets ofwater or an aqueous solution onto the zinc alloy plated steel sheet andthen using air, wherein when the droplets are sprayed, a droplet spraystart temperature ranges from 405° C. to 425° C., a droplet spray stoptemperature ranges from 380° C. to 400° C.

Advantageous Effects

According to one of various effects of the present disclosure, anembodiment of the present disclosure provides a zinc alloy plated steelsheet having high bending workability as well as high corrosionresistance.

In addition, according to one of various effects of the presentdisclosure, the zinc alloy plated steel sheet of the embodiment has highsurface quality.

In addition, according to one of various effects of the presentdisclosure, the zinc alloy plated steel sheet of the embodiment has highscratch resistance.

DESCRIPTION OF DRAWINGS

FIG. 1 is views illustrating results of (a) an observation of a surfacemicrostructure of Inventive Sample 1 and (b) an observation of a surfacemicrostructure of Comparative Sample 5.

FIG. 2 is views illustrating results of (a) an observation of across-sectional microstructure of Inventive Sample 1 and (b) anobservation of a cross-sectional microstructure of Comparative Sample 5.

FIG. 3 is a view illustrating results of X-ray diffractometer (XRD)analysis of Inventive Sample 1.

BEST MODE

Hereinafter, a zinc alloy plated steel sheet having high bendingworkability will be described in detail according to an aspect of thepresent disclosure.

According to the aspect of the present disclosure, the zinc alloy platedsteel sheet includes a base steel sheet and a zinc alloy plating layer.In the present disclosure, the base steel sheet is not limited to aparticular type. For example, a hot-rolled steel sheet or a cold-rolledsteel sheet commonly used as a base steel sheet of a zinc alloy platedsteel sheet may be used. However, hot-rolled steel sheets have a largeamount of surface oxide scale that lowers plating adhesion and thusplating quality, and thus a hot-rolled steel sheet from which oxidescale has been previously removed using an acid solution may be used asthe base steel sheet. In addition, the zinc alloy plating layer may beformed on one or each side of the base steel sheet.

The zinc alloy plating layer may include, by wt %, aluminum (Al): 0.5%to 3%, magnesium (Mg): 0.5% to 3%, and the balance of zinc (Zn) andinevitable impurities.

In the zinc alloy plating layer, magnesium (Mg) reacts with zinc (Zn)and aluminum (Al) and forms a Zn—Al—Mg-based intermetallic compound,thereby functioning as a key element improving the corrosion resistanceof the zinc alloy plated steel sheet. If the content of magnesium (Mg)is excessively low, the Zn—Al—Mg-based intermetallic compound is notpresent in sufficient amounts in the microstructure of the zinc alloyplating layer, and thus corrosion resistance may not be sufficientlyimproved. Therefore, the amount of magnesium (Mg) in the zinc alloyplating layer may be 0.5 wt % or greater, preferably 1.0 wt % orgreater. However, if the content of magnesium (Mg) is excessively high,the effect of improving corrosion resistance is saturated, and Mg oxidedross having a negative effect on platability may be formed in a platingbath. In addition, the Zn—Al—Mg-based intermetallic compound having highharness may be formed in excessively large amounts in the microstructureof the zinc alloy plating layer, and thus bending workability may belowered. Therefore, the amount of magnesium (Mg) in the zinc alloyplating layer may be 3 wt % or less, preferably 2.9 wt % or less.

Aluminum (Al) suppresses the formation of Mg oxide dross and reacts withzinc (Zn) and magnesium (Mg) to form the Zn—Al—Mg-based intermetalliccompound in the zinc alloy plating layer, thereby functioning as a keyelement improving the corrosion resistance of the zinc alloy platedsteel sheet. If the content of aluminum (Al) is excessively low, theformation of Mg dross is not sufficiently suppressed, and theZn—Al—Mg-based intermetallic compound is not present in sufficientamounts in the microstructure of the zinc alloy plating layer, which mayresult in insufficient improvements in corrosion resistance. Therefore,the amount of aluminum (Al) in the zinc alloy plating layer may be 0.5wt % or greater, preferably 0.6 wt % or greater. However, if the contentof aluminum (Al) is excessively high, the effect of improving corrosionresistance is saturated, and the durability of plating equipment may benegatively affected because of a high plating bath temperature.Moreover, the Zn—Al—Mg-based intermetallic compound having high harnessmay be formed in excessively large amounts in the microstructure of thezinc alloy plating layer, and thus bending workability may be lowered.Therefore, the amount of aluminum (Al) in the zinc alloy plating layermay be 3 wt % or less, preferably 2.6 wt % or less.

According to an embodiment, the contents of magnesium (Mg) and aluminum(Al) in the zinc alloy plating layer may satisfy the following Formula2. If [Mg]/[Al] is 1.0 or less, scratch resistance may deteriorate, andif [Mg]/[Al] is greater than 4.0, Mg-based dross may be formed in largeamounts in a hot-dip plating bath to lower workability.

1.0<[Mg]/[Al]≤4.0  [Formula 2]

where [Mg] and [Al] refer to the weight percentages (wt %) ofcorresponding elements, respectively.

The zinc alloy plating layer may include a Zn single phase structure asa microstructure and the Zn—Al—Mg-based intermetallic compound. In thepresent disclosure, the Zn—Al—Mg-based intermetallic compound is notlimited to a particular type. However, for example, the Zn—Al—Mg-basedintermetallic compound may include at least one selected from the groupconsisting of a Zn/Al/MgZn₂ ternary eutectic structure, a Zn/MgZn₂binary eutectic structure, a Zn/Al binary eutectic structure, and anMgZn₂ single phase structure.

The inventors have conducted in-depth research into improving thebending workability of zinc alloy plated steel sheets and found that ifa Zn single phase structure having a hexagonal close packing (HCP)structure is grown in a (0001) orientation in the microstructure of thezinc alloy plating layer, ductility increases owing to easy slippage,and thus cracks are markedly reduced in a bending process.

In the present disclosure, to obtain this effect, the degree (f) of(0001) preferred orientation, expressed by the following formula 1, maypreferably be adjusted to be 50% or greater, more preferably 60% orgreater.

f(%)=(I _(basal) /I _(total))×100  [Formula 1]

where I_(total) refers to the integral of all diffraction peaks of theZn single phase structure when an X-ray diffraction pattern is measuredwithin the range of 2 theta from 10° to 100° using a Cu-Kα source, andI_(basal) refers to the integral of diffraction peaks of the Zn singlephase structure relating to a basal plane.

In addition, the inventors have found that if the Zn single phasestructure coarsely formed in the zinc alloy plating layer is refined insize, it is also helpful to reduce cracking during a bending process.

To obtain this effect of the present disclosure, the average graindiameter of the Zn single phase structure may be preferably adjusted tobe 15 μm or less, more preferably 12 μm or less, and even morepreferably 10 μm or less. The “average grain diameter” of the Zn singlephase structure refers to the average of equivalent circular diametersof the Zn single phase structure measured by observing a thicknesswisecross-section of the zinc alloy plating layer.

The zinc alloy plated steel sheet of the present disclosure has highcorrosion resistance and bending workability as well.

According to an embodiment, the zinc alloy plated steel sheet of thepresent disclosure may have a good appearance. Specifically, the numberof black spots per unit area may be equal to or less than 0.1/cm² on thesurface of the zinc alloy plated steel sheet.

To obtain these effects of the present disclosure, the area fraction ofthe Zn single phase structure may preferably be 40% or less (excluding0%) on the surface of the zinc alloy plating layer. That is, theappearance of the zinc alloy plated steel sheet may be improved bymaximizing the fraction of the Zn—Al—Mg-based intermetallic compoundpresent on the surface of the zinc alloy plating layer.

According to an embodiment, the zinc alloy plated steel sheet of thepresent disclosure may also have high scratch resistance.

According to results of research conducted by the inventors, if the areafractions of the Zn/MgZn₂ binary eutectic structure and the Zn/Al/MgZn₂ternary eutectic structure which have a layer structure and are presenton the surface of the zinc alloy plating layer are maximized, scratchresistance may be markedly improved.

To obtain this effect of the present disclosure, preferably, the sum ofthe area fractions of the Zn/MgZn₂ binary eutectic structure and theZn/Al/MgZn₂ ternary eutectic structure may be 50% or greater (excluding100%), and the area fraction of the MgZn₂ single phase structure may be10% or less (including 0%). The MgZn₂ single phase structure has highhardness and thus causes cracks during a machining process, and thus thearea fraction of the MgZn₂ single phase structure may be adjusted to beas low as possible.

The zinc alloy plated steel sheet of the present disclosure may bemanufactured by various methods without limitation. However, forexample, when the zinc alloy plating layer solidifies from a moltenstate, the zinc alloy plating layer may be cooled by spraying dropletsthereon and then cooled with air to obtain the above-described degree ofpreferred orientation and average grain diameter.

In this case, droplets may be sprayed by a charge spray method to attachthe droplets by electrostatic attraction between the droplets and thezinc alloy plated steel sheet. This charge spray method may be helpfulin forming fine, uniform droplets and reducing the amount of dropletscolliding with and bouncing off the zinc alloy plated steel sheet afterbeing sprayed on the zinc alloy plated steel sheet, thereby facilitatingrapid cooling of the zinc alloy plating layer from the molten state andhaving a positive effect on the growth of the Zn single phase structurein the (0001) orientation and refinement of the Zn single phasestructure.

The droplets may be droplets of a phosphate aqueous solution capable ofrapidly cooling the zinc alloy plating layer from the molten statethrough an endothermic reaction and thus effective in growing the Znsingle phase structure in the (0001) orientation and refining the Znsingle phase structure. Examples of the phosphate aqueous solution mayinclude an aqueous solution of ammonium hydrogen phosphate ((NH₄)₂HPO₄),an aqueous solution of sodium ammonium hydrogen phosphate (NaNH₄HPO₄),an aqueous solution of zinc dihydrogen phosphate (Zn(H₂PO₄)₂), and anaqueous solution of calcium phosphate (Ca₃(PO₄)₂).

In addition, the content of the phosphate aqueous solution may be 1 wt %to 3 wt %. If the content of the phosphate aqueous solution is less than1 wt %, the effect of the phosphate aqueous solution may not besufficient. If the content of the phosphate aqueous solution is greaterthan 3 wt %, the effect of the phosphate aqueous solution is saturated,and nozzle clogging may occur in a continuous production process,lowering productivity.

In addition, when the droplets may be sprayed at a droplet spray starttemperature of 405° C. to 425° C., and more preferably 410° C. to 420°C. Here, the term “droplet spray start temperature” refers to a surfacetemperature of the zinc alloy plated steel sheet at the start time ofdroplet spraying. If the droplet spray start temperature is less than405° C., solidification of the Zn single phase structure may havealready started, and thus black spots may be formed on the surface ofthe zinc alloy plated steel sheet. Conversely, if the droplet spraystart temperature is greater than 425° C., droplets may not effectivelyundergo an endothermic reaction, and thus it may be difficult to obtainan intended structure.

In addition, the droplets may be sprayed at a droplet spray stoptemperature of 380° C. to 400° C., and more preferably 390° C. to 400°C. Here, the term “droplet spray stop temperature” refers to a surfacetemperature of the zinc alloy plated steel sheet at a point in time atwhich spraying of droplets stops. If the droplet spray stop temperatureis greater than 400° C., an endothermic reaction by the droplets mayoccur ineffectively, and thus it may be difficult to obtain an intendedstructure. Conversely, if the droplet spray stop temperature is lessthan 380° C., a Mg₂Zn₁₁ phase may be formed due to over cooling whilethe Zn/MgZn₂ binary eutectic phase and the Zn/Al/MgZn₂ ternary phasestart to solidify, and thus many black spots may be formed, decreasingthe degree of (0001) preferred orientation of the Zn single phasestructure.

In addition, the difference between the droplet spray start temperatureand the droplet spray stop temperature may be 15° C. or greater. If thedifference is less than 15° C., the droplets may not undergo aneffective endothermic reaction, and thus it may be difficult to obtainan intended structure.

In addition, the droplets may be sprayed in an amount of 50 g/m² to 100g/m². If the spraying amount of the droplets is less than 50 g/m², theeffect of the droplets may be insufficient, and if the spraying amountof the droplets is greater than 100 g/m², the effect of the droplets maybe saturated.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described more specificallythrough examples. However, the following examples should be consideredin a descriptive sense only and not for purpose of limitation. The scopeof the present invention is defined by the appended claims, andmodifications and variations reasonably made therefrom.

Example 1

Low carbon cold-rolled steel sheets each having a thickness of 0.8 mm, awidth of 100 mm, and a length of 200 mm were prepared as base steelsheets for plating test samples, and then foreign substances such asrolling oil were removed from the surfaces of the base steel sheets bydipping the base steel sheets into acetone and washing the base steelsheets with ultrasonic waves. Thereafter, a 750° C. reducing atmosphereheat treatment commonly performed to guarantee mechanicalcharacteristics of steel sheets in the hot-dipping plating field wasperformed on the base steel sheets, and then the base steel sheets weredipped into plating baths (bath temperature: 460° C.) havingcompositions shown in Table 1 below to fabricate zinc alloy plated steelsheets. Thereafter, each of the zinc alloy plated steel sheets was wipedwith gas to adjust a plating weight to be 70 g/m² on each side. Then,the zinc alloy plated steel sheets were cooled under the conditionsshown in Table 1 below and were cooled with air. Although not shown inTable 1 below, Comparative Sample 5 was prepared by performing a gaswiping process on a zinc alloy plated steel sheet fabricated using thesame plating bath as that used to fabricate Inventive Sample 1 to adjusta plating weight to be 70 g/m² on each side, and then cooling the zincalloy plated steel sheet using a general cooling device at an averagecooling rate of 12° C./sec until the plating layer of the zinc alloyplated steel sheet was completely solidified (at about 300° C. or less).

Then, the microstructures of the fabricated zinc alloy plated steelsheets were observed using an FE-SEM (SUPRA-55VP, Zeiss) as illustratedin FIGS. 1 and 2, and the average grain diameter of a Zn single phasestructure of each of the zinc alloy plated steel sheets was measured asshown in Table 2 below.

Thereafter, the degree (f) of (0001) preferred orientation of the Znsingle phase structure was measured using the following Formula 1, andresults thereof are shown in Table 2 below.

f(%)=(I _(basal) /I _(total))×100  [Formula 1]

where I_(total) refers to the integral of all diffraction peaks of theZn single phase structure when an X-ray diffraction pattern was measuredwithin the range of 2 theta from 10° to 100° using a Cu-Kα source, andI_(basal) refers to the integral of diffraction peaks of the Zn singlephase structure relating to a basal plane.

Thereafter, the bending workability of each of the zinc alloy platedsteel sheets was evaluated, and results thereof are shown in Table 2below.

Corrosion resistance was evaluated as follows. A salt spray test (basedon KS-C-0223) was performed on each of the zinc alloy plated steelsheets to facilitate corrosion, and then the time taken until the areafraction of red rust on the surface of each plating layer was 5% wasmeasured.

Bending workability was evaluated as follows.

3T bending was performed on each of the zinc alloy plated steel sheets,and a 1-mm length of the apex of each bent portion was observed using anSEM to measure the area fraction of bending cracks using an imageanalysis system.

TABLE 1 Composition of Droplet Droplet plating bath spray start spraystop Spraying (wt %) temperature temperature amount No. Al Mg (° C.) (°C.) Droplets (g/m²) Notes 1 1.6 1.6 410 390 Aqueous solution 70 *IS 1 ofammonium hydrogen phosphate, 2 wt % 2 1.6 1.6 420 400 Aqueous solution70 IS 2 of ammonium hydrogen phosphate, 2 wt % 3 1.6 1.6 430 400 Aqueoussolution 70 **CS 1 of ammonium hydrogen phosphate, 2 wt % 4 1.6 1.6 400390 Aqueous solution 70 CS 2 of ammonium hydrogen phosphate, 2 wt % 51.6 1.6 420 405 Aqueous solution 70 CS 3 of ammonium hydrogen phosphate,2 wt % 6 1.6 1.6 410 375 Aqueous solution 70 CS 4 of ammonium hydrogenphosphate, 2 wt % *IS: Inventive Sample, **CS: Comparative Sample

TABLE 2 Average grain diameter of Zn Red rust Area fraction of singlephase occurrence bending cracks No. structure (μm) f (%) time (h) (%)Notes 1 8 63 650 8 *IS 1 2 10 62 645 9 IS 2 3 12 49 640 25 **CS 1 4 1447 630 38 CS 2 5 15 46 620 40 CS 3 6 16 44 610 42 CS 4 7 18 42 600 45 CS5 *IS: Inventive Sample, **CS: Comparative Sample

Referring to Table 2, Inventive Samples 1 and 2 satisfying conditionsproposed in the present disclosure had high bending workability.

However, although Comparative Samples 1 to 5 had high corrosionresistance, Comparative Samples 1 to 5 had poor bending workabilitybecause the (f) values thereof were less than 50%.

FIG. 1 is views illustrating results of (a) an observation of a surfacemicrostructure of Inventive Sample 1 of the present disclosure and (b)an observation of a surface microstructure of Comparative Sample 5, andFIG. 2 is views illustrating results of (a) an observation of across-sectional microstructure of Inventive Sample 1 of the presentdisclosure and (b) an observation of a cross-sectional microstructure ofComparative Sample 5.

FIG. 3 is a view illustrating results of X-ray diffractometer (XRD)analysis of Inventive Sample 1. In FIG. 3, peaks denoted with “◯” and“●” are all diffraction peaks of the Zn single phase structure, and thepeaks denoted with “◯” are diffraction peaks of the Zn single phasestructure relating to a basal plane.

Example 2

Low carbon cold-rolled steel sheets each having a thickness of 0.8 mm, awidth of 100 mm, and a length of 200 mm were prepared as base steelsheets for plating test samples, and then foreign substances such asrolling oil were removed from the surfaces of the base steel sheets bydipping the base steel sheets into acetone and washing the base steelsheets with ultrasonic waves. Thereafter, a 750° C. reducing atmosphereheat treatment commonly performed to guarantee mechanicalcharacteristics of steel sheets in the hot-dipping plating field wasperformed on the base steel sheets, and then the base steel sheets weredipped into plating baths having compositions shown in Table 3 below tofabricate zinc alloy plated steel sheets. Thereafter, each of the zincalloy plated steel sheets was wiped with gas to adjust a plating weightto be 70 g/m² on each side. Then, the zinc alloy plated steel sheetswere cooled under the same conditions as Inventive Sample 1 of Example1.

Thereafter, the fractions of microstructures observed on the surface ofeach of the zinc alloy plated steel sheets were measured, and the numberof black spots on the surface of each of the zinc alloy plated steelsheets was measured. Results thereof are shown in Tables 3 and 4.

Thereafter, a friction test (linear friction test) was performed byrubbing the surface of each of the zinc alloy plated steel sheets 20times with a tool head at a constant pressure. In the friction test, atarget load was 333.3 kgf, a pressure was 3.736 MPa, the tool headtraveled 200 mm per rub, and the speed of the tool head was 20 mm/s.

After the friction test, a stripping test was performed on each of thezinc alloy plated steel sheets. Specifically, cellophane adhesive tape(NB-1 by Ichiban) was attached to a bent portion of each of the zincalloy plated steel sheets subjected to a 10R bending process, and thenthe cellophane tape was momentarily separated. Then, the number ofplating layer defects was measured using an optical microscope(magnification: 50 times). Results of the measurement were evaluated as“◯” when the number of plating layer defects was 5/m² or less, and “X”when the number of plating layer defects was greater than 5/m².Evaluation results are shown in Table 4 below.

In addition, after the friction test, each of the zinc alloy platedsteel sheets was inserted into a salt spray tester, and the time takenuntil the occurrence of red rust was measured according to internationalstandard ASTM B117-11. In that time, a 5% salt solution (35° C., pH 6.8)was sprayed at a rate of 2 ml/80 cm² per hour. When the time taken untilred rust was present on a sample was 500 hours or greater, the samplewas evaluated as “◯”, and when the time taken until red rust was presenton a sample was less than 500, the sample was evaluated as “X.” Resultsof the evaluation are shown in Table 4 below.

TABLE 3 Alloy composition Area fractions of surface structures (area %)(wt %) Zn/Al/MgZn₂ + No. Al Mg Mg/Al Zn Zn/MgZn₂ Zn/Al/MgZn₂ MgZn₂ Zn/AlZn/MgZn₂ Notes 1 0.6 2.3 3.83 28 41 31 0 0 72 *IS A 2 1.5 2.8 1.87 20 5721 1 1 78 IS B 3 2 2.9 1.45 8 63 28 1 0 91 IS C 4 2.2 2.7 1.23 4 58 34 22 92 IS D 5 2.6 2.9 1.12 4 39 51 3 3 90 IS E 6 0 0 — 100 0 0 0 0 0 **CSA 7 1.4 1 0.71 82 7 11 0 0 18 CS B 8 2.5 1.2 0.48 6 21 26 46 1 47 CS C 95 0 0.00 76 0 0 0 24 0 CS D 10 5 1 0.20 59 9 11 0 21 20 CS E 11 8 3 0.3813 7 13 18 49 20 CS F 12 55 0 0.00 14 0 0 0 86 0 CS G Here, surfacestructures refer to microstructures observed on the surfaces of zincalloy plating layers. *IS: Inventive Sample, **CS: Comparative Sample

TABLE 4 Results of Results of stripping salt spray test after test afterNumber friction test friction test of black Number Evalua- Red RustEvalua- spots of defects tion Occurrence tion No. (/cm²) (/m²) resultstime (hours) results Notes 1 0.05 3 ◯ 520 ◯ *IS A 2 0.08 2 ◯ 550 ◯ IS B3 0.04 4 ◯ 600 ◯ IS C 4 0.08 3 ◯ 650 ◯ IS D 5 0.04 2 ◯ 580 ◯ IS E 6 1.22 ◯ 120 X **CS A 7 0.8 3 ◯ 230 X CS B 8 0.05 23 X 620 ◯ CS C 9 1.1 3 ◯350 X CS D 10 0.6 2 ◯ 420 X CS E 11 0.06 15 X 650 ◯ CS F 12 0.05 11 X200 X CS G *IS: Inventive Sample, **CS: Comparative Sample

Referring to Table 4, Inventive Samples A to E satisfying conditionsproposed in the present disclosure had good appearance and high scratchresistance.

However, each of Comparative Samples A, B, D, and E had poor appearancebecause the area fraction of a Zn single phase structure present on thesurface of a plating layer was excessively high, and each of ComparativeSamples A to G had poor scratch resistance because the area fractions ofa Zn/MgZn₂ binary eutectic structure and a Zn/Al/MgZn₂ ternary eutecticstructure are excessively low.

1. A zinc alloy plated steel sheet comprising a base steel sheet and a zinc alloy plating layer, wherein the zinc alloy plating layer comprises a Zn single phase structure as a microstructure and a Zn—Al—Mg-based intermetallic compound, and the Zn single phase structure has a degree (f) of (0001) preferred orientation, expressed by Formula 1 below, within a range of 50% or greater, f(%)=(I _(basal) /I _(total))×100  [Formula 1] where I_(total) refers to an integral of all diffraction peaks of the Zn single phase structure when an X-ray diffraction pattern is measured within a range of 2 theta from 10° to 100° using a Cu-Kα source, and I_(basal) refers to an integral of diffraction peaks of the Zn single phase structure relating to a basal plane.
 2. The zinc alloy plated steel sheet of claim 1, wherein the Zn single phase structure has a degree (f) of (0001) preferred orientation, expressed by Formula 1, within a range of 60% or greater.
 3. The zinc alloy plated steel sheet of claim 1, wherein the Zn—Al—Mg-based intermetallic compound comprises at least one selected from the group consisting of a Zn/MgZn₂ binary eutectic structure, a Zn/Al binary eutectic structure, an MgZn₂ single phase structure, and a Zn/Al/MgZn₂ ternary eutectic structure.
 4. The zinc alloy plated steel sheet of claim 1, wherein an area fraction of the Zn single phase structure on a surface of the zinc alloy plating layer is 40% or less (excluding 0%).
 5. The zinc alloy plated steel sheet of claim 1, wherein a total area fraction of a Zn/MgZn₂ binary eutectic structure and a Zn/Al/MgZn₂ ternary eutectic structure is 50% or greater (excluding 100%) on a surface of the zinc alloy plating layer.
 6. The zinc alloy plated steel sheet of claim 1, wherein an area fraction of an MgZn₂ single phase structure on a surface of the zinc alloy plating layer is 10% or less (excluding 0%).
 7. The zinc alloy plated steel sheet of claim 1, wherein an average grain diameter of the Zn single phase structure observed on a cross-section of the zinc alloy plating layer taken in a sheet thickness direction is 15 μm or less (excluding 0 μm).
 8. The zinc alloy plated steel sheet of claim 1, wherein the zinc alloy plating layer comprises, by wt %, aluminum (Al): 0.5% to 3%, magnesium (Mg): 0.5% to 3%, and a balance of zinc (Zn) and inevitable impurities.
 9. The zinc alloy plated steel sheet of claim 1, wherein the zinc alloy plating layer satisfies Formula 1 below: 1.0<[Mg]/[Al]≤4.0  [Formula 2] where [Mg] and [Al] refer to weight percentages (wt %) of corresponding elements, respectively.
 10. The zinc alloy plated steel sheet of claim 1, wherein a number of black spots per unit area is 0.1/cm² or less on a surface of the zinc alloy plated steel sheet.
 11. A method for manufacturing a zinc alloy plated steel sheet, the method comprising: preparing a zinc alloy plating bath comprising magnesium (Mg) and aluminum (Al); obtaining a zinc alloy plated steel sheet by dipping a base steel sheet into the zinc alloy plating bath to plate the base steel sheet; wiping the zinc alloy plated steel sheet with gas to adjust a plating weight; and after adjusting the plating weight of the zinc alloy plated steel sheet, cooling the zinc alloy plated steel sheet by spraying droplets of water or an aqueous solution onto the zinc alloy plated steel sheet and then using air, wherein when the droplets are sprayed, a droplet spray start temperature ranges from 405° C. to 425° C., a droplet spray stop temperature ranges from 380° C. to 400° C.
 12. The method of claim 11, wherein when the droplets are sprayed, a difference between the droplet spray start temperature and the droplet spray stop temperature is 15° C. or greater.
 13. The method of claim 11, wherein the droplets are sprayed by a charge spray method to attach the droplets by electrostatic attraction between the droplets and the zinc alloy plated steel sheet.
 14. The method of claim 11, wherein the droplets are sprayed in an amount of 50 g/m² to 100 g/m².
 15. The method of claim 11, wherein the aqueous solution is a phosphate aqueous solution.
 16. The method of claim 15, wherein the phosphate aqueous solution comprises at least one selected from the group consisting of an aqueous solution of ammonium hydrogen phosphate ((NH₄)₂HPO₄), an aqueous solution of sodium ammonium hydrogen phosphate (NaNH₄HPO₄), an aqueous solution of zinc dihydrogen phosphate (Zn(H₂PO₄)₂), and an aqueous solution of calcium phosphate (Ca₃(PO₄)₂).
 17. The method of claim 15, wherein the phosphate aqueous solution has a concentration of 0.5 wt % to 5 wt %.
 18. The method of claim 11, wherein the zinc alloy plating bath comprises, by wt %, aluminum (Al): 0.5% to 3%, magnesium (Mg): 0.5% to 3%, and a balance of zinc (Zn) and inevitable impurities. 